From df6455130d91c3987c7ce59dc51b0fbd16110524 Mon Sep 17 00:00:00 2001
From: jmcarcell <jmcarcell@users.noreply.github.com>
Date: Fri, 15 Sep 2023 10:36:26 +0200
Subject: [PATCH] Check if the CLHEP target exists before creating it, if it
 does warn

---
 cmake/DD4hepBuild.cmake | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/cmake/DD4hepBuild.cmake b/cmake/DD4hepBuild.cmake
index ab12d75a7..bcd3407f7 100644
--- a/cmake/DD4hepBuild.cmake
+++ b/cmake/DD4hepBuild.cmake
@@ -826,7 +826,11 @@ MACRO(DD4HEP_SETUP_GEANT4_TARGETS)
 
     if(Geant4_builtin_clhep_FOUND)
       dd4hep_debug("Using Geant4 internal CLHEP")
-      ADD_LIBRARY(CLHEP::CLHEP INTERFACE IMPORTED GLOBAL)
+      if(TARGET CLHEP::CLHEP)
+        message(WARNING "CLHEP::CLHEP already exists, this may cause problems if there are two different installations of CLHEP, one from Geant4 and one external")
+      else()
+        ADD_LIBRARY(CLHEP::CLHEP INTERFACE IMPORTED GLOBAL)
+      endif()
       SET_TARGET_PROPERTIES(CLHEP::CLHEP
         PROPERTIES
         INTERFACE_INCLUDE_DIRECTORIES "${Geant4_INCLUDE_DIRS}"